CO2 -Induced in-situ gelation of polymeric viscosifiers for permeability contrast correction

ABSTRACT

A water permeability contrast correction process to improve the sweep efficiency of waterflooding which involves the sequential injection of (1) an optional aqueous preflush slug to adjust connate water salinity, (2) an aqueous thickened slug comprising a sequestered polyvalent metal cation such as aluminum citrate and a gelable polymeric viscosifier such as polyacrylamide, (3) carbon dioxide to decrease the pH of the polymer slug which triggers the delayed in-situ gelation of said thickened slug to preferentially decrease water permeability in highly permeable thief zones, and (4) an aqueous drive fluid.

The invention pertains to a method for correcting the water permeabilitycontrast of heterogeneous subterranean formations. In one aspect, theinvention pertains to permeability contrast correction of undergroundstrata. In a particular aspect, the invention pertains to the selectiveplugging of more permeable strata of subterranean formations by theinjection therein of gelable aqueous polymer solutions followed by theinjection of a gel-triggering gaseous component to effect in-situgelation. In another aspect, the invention pertains to the creation ofgel plugs in subterranean formations under controlled conditions. In afurther aspect, the invention pertains to methods of waterflooding.

BACKGROUND OF THE INVENTION

Methods of water diversion employed in waterflooding depend in part onthe degree of heterogeneity of the porous media or strata being treated.Relatively more permeable zones of the subterranean formation tend totake most of the injected fluids. Initially, this is acceptable insweeping oil from such zones of relatively high permeability. However,this subsequently becomes undesirable as the oil content of such stratabecomes depleted since much of the subsequently injected flood water orother fluid by-passes the relatively less permeable but oil-bearingzones and provides little benefit in enhancing further hydrocarbonrecovery.

An isolated high-permeability zone or fracture can be plugged at thewell core face by a shallow layer of applied cement, though such apermanent relatively irrevocable technique often is consideredundesirable. More desirably, a communicating high-permeability zonepreferably is plugged to some considerable outward lateral depth to bemost effective in preventing floodwater or gas from otherwise merelyflowing around a narrow shallow plug and back into the high-permeabilityor swept zone. In depth plugging of a relatively high-permeability zoneconverts the zone into a zone of much lower permeability. Subsequentlyinjected floodwater or other fluid then will tend to enter formerlyby-passed but now relatively more-permeable hydrocarbon-bearing zonesand thus mobilize increased amounts of hydrocarbons therefrom.

In depth plugging can be effected by the injection of gelable thickenedaqueous solutions containing sequestered polyvalent metal cations whichcause the gelation or crosslinking of the thickened aqueous solutionswhen the pH of the solution is in a gelation pH range.

The injection of gelable systems triggered by a following aqueous acidicsolution for in-situ pH adjustment has been used. However, this sequencemay result in gelation occurring so rapidly and shallowly that asufficient lateral outward depth of plugging is not effectively obtainedin the most permeable strata where needed. On the other hand, when theacidic component is premixed with the gelable composition, gelation alsocan be too swift, resulting in the necessity of shearing the gelledpolymer in order to be able to obtain adequate injection, but whichreduces the effectiveness of the gel.

Needed is a method of in-situ gelling a gelable injectable aqueousliquid composition which has an initial pH outside the gelation pH rangeyet which possesses the capability of forming the desired gel in-situ,and without the need for premixing with acid or the follow-up injectionof acid.

BRIEF DESCRIPTION OF THE INVENTION

We have discovered an in-situ gelling method employing the injection ofcarbon dioxide subsequent to the injection of a gelable composition intoa subterranean formation. The carbon dioxide slug adjusts the pH of thegelable composition into a pH range effective for in-situ gelation totake place. The compositions employed comprise water, a water-thickeningamount of a polymer capable of gelling in the presence of a crosslinkingagent, and a suitable amount of crosslinking agent capable of gellingthe polymer. The subsequently injected effective amount of carbondioxide triggers the in-situ gelation of the gelable compositions.

The components, other than the carbon dioxide, are admixed in a liquidstate at a pH above the gelation pH range, thus in a non-gelled state.After the injected composition has penetrated into an undergroundformation and positioned itself, gelation is triggered in-situ by thefingering of the very highly mobile carbon dioxide which tends to flowpreferentially into the highly permeable strata so as to reduce the pHtherein into the gelation pH range, thereby effecting in depth pluggingof the strata with the now-gelled gelable composition.

While some preferential placement of the pre-gel material occurs due tothe contrast of permeability between the high and low permeabilitystreaks, the relatively high viscosity of some of the pre-gel aqueousinjection material tends to lead in some instances to a more uniformpenetration than desired. Only fractures and extremely high permeabilitystreaks will show significant preferential filling. This is true of mostof the bulk gelling systems since they usually involve injection ofrelatively viscous materials.

The primary feature of our invention which results in better placementselectivity than all liquid systems is the use of the gas phase trigger.The gas has a very low viscosity which gives it a great tendency tofinger through the more permeability streaks, even when there is arelatively small permeability contrast. With the high fingering tendencyof our triggering material, gel forms only where the gas goes, which isprecisely where it is needed.

The ungelled polymer in the low permeability zones then is displaced bythe following flood waters. Some of this pre-gel material is displacedback into the high permeability streak beyond the gel plug and is gelledthere because of the lower pH remaining from the effects of the previousCO₂ flush. A second factor in gel placement with gas is the tendency forthe gas slug to push some polymer solution ahead of it as it fingersthrough the high permeability streak.

In a water permeability contrast correction process (sometimes termed inprevious disclosures water permeability correction process) to improvethe sweep efficiency of waterflooding operations, the following sequenceof injections is used: (1) an optional aqueous preflush slug to adjustconnate water salinity; (2) a thickened aqueous slug comprising asequestered polyvalent metal cation, such as aluminum citrate, and agelable polymeric viscosifier, such as partially hydrolyzedpolyacrylamide; (3) carbon dioxide in amounts suitable for decreasingthe pH of the priorly injected thickened aqueous slug (2) so as totrigger the in-situ gelation of the thickened slug, thus effectingpreferential decrease in water permeability of the highly permeablethief zones; and (4) an aqueous drive fluid.

Our process has the advantage of employing an aqueous composition whichitself is non-acidic and uses non-acidic materials which are relativelyeasily handled and stored. The aqueous admixtures for injection are atan initial pH above about 7.5, thus they are relatively non-corrosiveand easily handled without effecting or causing undue deterioration ofpipeline, pumps, casings, and the like.

Our invention provides a method for the in-situ plugging of highpermeability zones in hydrocarbon bearing formations. A non-acidicthickened aqueous slug containing complexed polyvalent metalcrosslinking agent at an alkaline pH is injected into a heterogeneousformation. This slug has the tendency to flow predominantly into thehigh permeability zones. Thereafter, carbon dioxide, or carbon dioxidemixed with other gases, is injected and fingers its way spontaneouslyinto the high permeability zones to contact the previously injectedthickened aqueous slug. Prior to the arrival of the carbon dioxide inthese zones, the thickened aqueous slug remains ungelled. Afterinjection of the carbon dioxide into the formation, some of the carbondioxide dissolves in the reservoir brine and in the thickened aqueousslug, thereby lowering the pH of the thickened aqueous slug sufficientlyto activate the in-situ gelation of the slug by the sequesteredpolyvalent metal crosslinking agent. This method, of our invention,allows the thickened aqueous slug to be placed far out into thesubterranean formation before gelation is initiated by the CO₂ describedpH-adjustment technique. The highly mobile gaseous CO₂ channels veryrapidly into the highly permeable zones and therein triggers gelation bylowering the pH in the body of the previously injected gelablecomposition.

It is an object of our invention to provide an in depth permeabilitycontrast correction method for plugging the more permeable strata insubterranean formations. It is also an object of our invention toprovide a method for treating an underground formation in which theplugging is delayed by controlling the pH of the injected liquids untilthe fluids have been positioned deep within the heart of the permeableunderground formation. It is a further object of our invention toprovide a method for employing gelable injectable liquid compositionswhich have a pH outside the gelation pH range yet possess the capabilityof forming the desirable gels in-situ when contacted by a followinginjection of carbon dioxide as a gel-triggering reagent.

Other aspects, objects, and the various advantages of this inventionwill become apparent upon reading this specification and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates by means of a series of drawings A, B, C, and D apictorial sequence of gel placement. The well bore face is indicated by10.

(A) Initially less permeable stratum is shown 11, and initially morepermeable stratum 12. The pre-gel, aqueous gelable admixture 13 movesinto the several strata 11, 12, forming a front 14.

(B) Upon CO₂ injection 15, acidification and gelation 16 occurs only inthe initially more permeable strata 12 since the CO₂ fingers mostreadily therethrough.

(C) Floodwater 17 then is applied, and some pre-gel in the ungelledstrata 11 is pushed along, and some likely tends to pass plug 16 andwash 18 into the zone of more permeability, meeting low pH aqueous CO₂19.

(D) The above effect is to form a further gel plug 21 extent.

FIG. 2 illustrates (a) cross-section and (b) longitudinal views of asimulated sand pack prepared with an internal core 1 of fine glass leadssurrounded by 2 slightly larger glass beads. The simulated sand packshown split longitudinally illustrates by flow arrows 3 the tendency ofa liquid to flow in the more permeable inner core of coarse beads as thepath of relative least-resistance.

FIG. 3 illustrates the same simulated sand pack as in FIG. 2, butreflects the effects of our CO₂ in-situ gelation treatment. The gel 4formed in the initially more permeable coarse core (1), effectivelymaking it now less permeable, such that following flood water (arrows 3)now moves through the formerly less permeable but now more permeablefine beads (2).

FIG. 4 illustrates a further run similar to FIG. 3, with the gel spacedor placed outwardly somewhat.

FIG. 5 illustrates the gelation of a polymer solution, exemplified by analuminum citrate/polyacrylamide solution, as a function of pH,illustrating the dramatic gelling occurring within a pH range of about 3to 7.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with our invention, the injection of the gelablecomposition is followed by the injection of an effective pH-adjusting(pH-lowering) amount of carbon dioxide. The gelable compositioncomprising water, a water-thickening amount of a water-soluble orwater-dispersible polymeric viscosifier, and a polyvalent metal cationcrosslinking agent. The pH of the injected composition is adjustablein-situ. The carbon dioxide under the conditions of temperature andpressure of the underground strata reduces the pH of the injectedaqueous composition into the gelation range, thus triggering the in-situgelation of the injected aqueous gelable composition.

In one embodiment of our invention, a plug is created in an undergroundformation by injecting into the formation an admixture comprising water,a gelable polymer, and a crosslinking agent, with an admixture pHoutside the gelation range, followed by carbon dioxide which fingerspreferentially into the highly permeable strata and therein reduces thepH of the injected admixture into the gelation range, thereby triggeringin-situ gelation of the placed admixture.

Polymers

Polymers suitable for use in our invention are those capable of gellingin the presence of polyvalent metal ion crosslinking agents within agelation pH range. Suitable polymers include biopolysaccharides,cellulose ethers, and acrylamide-based polymers.

Suitable cellulose ethers are disclosed in U.S. Pat. No. 727,688(incorporated herein by reference). Particularly preferred celluloseethers include carboxymethylhydroxyethyl cellulose (CMHEC) andcarboxymethyl cellulose (CMC).

Suitable biopolysaccharides are disclosed in U.S. Pat. No 4,068,714(incorporated herein by reference). Particularly preferred ispolysaccharide B-1459 which is a biopolysaccharide produced by theaction of Xanthomonas campestris bacteria. This biopolysaccharide iscommercially available in various grades under the trademark Kelzan™(Kelco Company, Los Angeles, Calif.).

Suitable acrylamide-based polymers are disclosed in U.S. Pat. No.3,749,172 (incorporated herein by reference). Particularly preferred arethe so-termed partially hydrolyzed polyacrylamides possessing pendantcarboxylate groups through which crosslinking can take place. Thermallystable copolymers of acrylamide, such aspoly(N-vinyl-2-pyrrolidone-co-acrylamide) andpoly(sodium-2-acrylamido-2-methyl-1-propanesulfonate-co-acrylamide), areparticularly suitable for applications in high salinity environments atelevated temperatures. Various terpolymers also are useful in thepresent process, such as terpolymers derived from acrylamide andN-vinyl-2-pyrrolidone comonomers with lesser amounts of termonomers suchas vinyl acetate, vinylpyridine, styrene, methyl methacrylate, and thelike.

Other miscellaneous polymers suitable for use in the present inventioninclude partially hydrolyzed polyacrylonitriles, polystyrene sulfonates,lignosulfonates, methylolated polyacrylamides, and the like.

In general, the gelation pH range is within a pH range of about 3 to 7.It is recognized that this range may vary somewhat for various polymers,or for polymer-polyvalent metal cation combinations or concentrationrelationships. The specific gelation pH range is readily determinablefor a given or specific polymer or polymer-polyvalent metal cationcombination by testing same with additions of acid and following theresulting pH change until gelation is observed.

Presently preferred are the acrylamide based polymers, particularly thepolyacrylamides and the partially hydrolyzed polyacrylamides, preferablyin conjunction with Al³⁺ as the polyvalent metal cation, presently mostpreferably as the aluminum citrate complex.

The concentration or water-thickening amount of thewater-soluble/dispersible polymer in the aqueous solution/dispersion canrange widely and be as suitable and convenient for the various polymers,and for the degree of gelation needed for particular strata. Generally,the concentration of polymer in its aqueous solution/dispersion (beforeadmixing with crosslinking components) is about 1,000 to 20,000 ppm,preferably about 2,000 to 5,000 ppm.

Any suitable procedures for preparing the aqueous admixtures of thecrosslinkable polymer can be used. Some polymers may require particularmixing conditions, such as slow addition of finely powdered polymer intothe vortex of stirred water, alcohol pre-wetting, protection from air(oxygen), preparation of stock solutions from fresh rather than saltwater, or the like, as is known for such polymers.

Crosslinking Agents

The crosslinking agents are multivalent (polyvalent) metal cations whichare effective for gelling the selected polymer when the aqueousadmixture is within the pH gelation range. Polyvalent metal ionsPreferably are one or more of Fe²⁺, Fe³⁺, Al³⁺, Ti⁴⁺, Zn²⁺, Sn⁴⁺, Ca²⁺,Mg²⁺, and Cr³⁺. Presently preferred are Al³⁺ and Cr³⁺ ; although Cr³⁺ ispresently preferred in high brines.

The polyvalent metal ions are employed in the form of compounds orcomplexes with an effective sequestering or chelating amount of one ormore chelating or sequestering anions. The chelating or sequesteringanions typically are one or more of acetate, nitrilotriacetate,tartrate, citrate, tripolyphosphate, metaphosphate, gluconate,phosphate, and the like, including mixtures thereof. The chelating orsequestering agent retards the onset and rate of gelation of thepolymer. Presently preferred is citrate.

The solution of polyvalent metal cation is prepared from a suitablewater-soluble compound of the metal, such as the sulfate, chloride,nitrate, acetate, and the like, by admixing the metal compound withsufficient amounts of water to make up a desired or convenient stocksolution concentration such as about 1 to 5 weight percent of polyvalentmetal compound. Fresh water is preferred for best solubilitycharacteristics.

The sequestering agent usually is supplied or available as the sodiumsalt, or in some cases as the free acid. A stock solution is made up inwater, preferably fresh water, to a convenient concentration, such asabout 1 to 5 weight percent of sequestering agent.

The polyvalent metal salt solution and the sequestering agent solutionthen are admixed in suitable proportions to result in the desiredsequestered polyvalent metal cation solution. The amount of chelating orsequestering anion employed presently preferably is sufficient tosubstantially associate with the polyvalent metal cation present in thesolution. Generally, the molar ratio of polyvalent metal cation tosequestering anion varies over the broad range of about 1:1 to 6:1,preferably about 2:1 to 4:1, and presently most preferably about 2.5:1to 3.5:1.

Conveniently, the polymer can be dispersed in a given amount of water,preferably fresh water, and to it then added the desired amounts of asolution or dispersion of the sequestered polyvalent metal cationcrosslinking agent. The amount of crosslinking agent used dependslargely on the amounts of polymer solution. Lesser amounts of polymerrequire lesser amounts of crosslinking agent, and vice-versa. Further,it has been found that for a given concentration of polymer thatincreasing the amount of crosslinking agent generally substantiallyincreases the rate of crosslinking. Generally, the ratio of polyvalentmetal cations to crosslinkable side groups of the polymeric viscosifierwill vary over the broad range of about 10:1 to 1:10, preferably about5:1 to 1:1.

After admixing with the aqueous solution of the crosslinking agent, thepolymer concentration generally will be on the order of about 500 to10,000 ppm, more usually about 1,000 to 2,500 ppm.

Compositions for Injection

The pre-injection pH of the gelable compositions prepared for injectionis critical in order to achieve effective plugging of the undergroundformation. The thickened aqueous compositions prepared at the surfaceexhibit pH values of about 7.5 or above. The effective amount of carbondioxide subsequently added thereto, below ground, gradually, under theconditions of heat and pressure existing in the strate, fingerseffectively into the injected gelable composition to lower its pH intothe gelation pH range, thus triggering gelation deep in the permeablestrata.

Amount of In-Situ Gelable Composition

The total quantity of in-situ-gelable aqueous treating compositionemployed can be expressed in terms of the pore volume of the stratumarea to be treated. For example, if a region (one or more stratum orportion thereof) to be treated is taking upwards of perhaps about 80volume percent of the injected fluid, a packer can be set to direct thetreating composition into this zone. The quantity of treatingcomposition can vary widely, depending on the effects desired, butgenerally in the range of about 100% to 120% of the pore volume of thezone to be treated with the upper limit being governed mainly by thepractical limitations of pumping expense and chemical costs.

Carbon Dioxide

As the next step, in our invention, after injection of the gelablecomposition, i.e., the thickened aqueous composition admixture, into theheterogeneous subterranean formation, carbon dioxide is injected.

The highly mobile carbon dioxide fingers preferentially into the highlypermeable strata, becomes admixed therein with the previously placedgelable composition, dissolves in the water, lowers the pH into the pHgelation range, resulting in gel formation.

The carbon dioxide can be conveniently employed as a gas, suitablycompressed to overcome necessary downhole back pressure injectionrequirements. Alternatively, the carbon dioxide can be employed andinjected as a liquid under suitable temperature and pressure control.Injection as a liquid into relatively hot formations may at times bedesirable, since heat of vaporization can be readily obtained from theformation.

Carbon dioxide can be employed in gaseous admixture with such asnitrogen, steam, or mixtures thereof, if desired or convenient dependingon the available source of the carbon dioxide. Carbon dioxide can beemployed as saturated or super saturated admixtures with water. Steamgenerators can be employed wherein the fuel and air are combusted andthe gases then used for injection purposes. Once-through steamgenerators also can be used since the product therefrom is an admixtureprimarily of steam, carbon dioxide, and nitrogen.

In some areas, carbon dioxide or carbon dioxide-containing gases areavailable from such as CO₂ -producing wells, CO₂ -natural gas admixturesfrom which the natural gas is separated leaving available the CO₂, stackgases, and the like.

The amount of carbon dioxide to employ is that amount needed to effectthe necessary pH adjustment resulting in the degree of in-situ gelformation and flow impedance desired. This depends on the pH gelationrange of the gel-forming materials chosen, i.e., the polymer(s) andpolyvalent metal ion(s). Above-ground tests on the gelation admixture,i.e., the thickened aqueous composition, can be readily made byinjecting CO₂ into a sample and monitoring the gelation pH and the CO₂requirements. Additionally, or more generally in actual practice,reliance is placed on monitoring back-pressure and injection-resistancerequirements down-hole so as to judge appropriate cessation of carbondioxide injection.

Since carbon dioxide is at least partially water-soluble, and such watersolubility tends to increase with increasing pressures, it is expectedthat at high reservoir pressures the carbon dioxide may be somewhat slowin initially propagating outwardly into the formation because ofdissolution, at least until connate water reservoir pressure demands aresatisfied.

The injected carbon dioxide tends to flow in a fingering fashion andflows most readily through the zones of relatively high permeability andthus contacting the in-place thickened aqueous slug in zones where mostneeded.

It presently is preferred to use an inert and less soluble carrier gas,such as nitrogen, so that the injected gas phase can propagate itselffurther outwardly into the reservoir and into the placedgelable-solution admixture. With such a carrier gas, it is believed thatthe otherwise somewhat adverse mobility ratio of the carbon dioxidealone is improved and this improves formation of an maintenance of anextended gelled band in the (now-less) permeable zone(s).

Subsequent De-Gelation

It sometimes is desirable to plug a portion of an underground formationso as to divert subsequently injected treating fluids into certain zonesof the formation and yet be able to remove the stoppages from theformation after the particular treatment is completed. Gel-formingcompositions are advantageous, since gel-breaking can be accomplished,when desired, by appropriate subsequent treatment.

The plug can be substantially reduced or eliminated following thegelation at any time convenient thereafter by injecting an agent such assodium hypochlorite which is recognized in the art for its effectivenessin degrading polymeric viscosifiers such as the polyacrylamides, whichupon contacting the gel-plug in the formation gradually causesdissolution thereof.

Preflush (Optional)

Prior to employment of the gelable compositions, the strata can besubjected to a conditioning preflush step.

The optional preflush employs an aqueous solution with a lower level ofhardness (calcium and magnesium ions) and/or total dissolved solids(tds) than that of the stratum connate water, and preferably containingsubstantially no hardness cations though it may be saline. The purposeof the preflush is to alter the salinity of the connate water byflushing the formation, generally with about one to three times the porevolume of the zone to be treated.

Since it is known that enhanced oil recovery chemicals such assurfactants and polymeric viscosifiers are adsorbed and/or precipitatedto a greater extent in the presence of electrolytes and hardness cationsin particular, the preflush alleviates this potential problem bysweeping out at least a fraction of such electrolytes. A typical NaClpreflush brine contains, e.g., on the order of about 0.2 to 2 weightpercent total dissolved solids.

Sodium chloride is the generally preferred salt although other sodiumsalts such as the nitrate, sulfate, acetate, and the like, are suitable.It is contemplated that even more concentrated salt solutions can beused for preflushing provided that no incompatibility results with thedissolved polymeric viscosifier such as said polyacrylamide.

The preflush may contain polymeric thickeners such as partiallyhydrolyzed polyacrylamides and conventional sacrificial agents such assodium carbonate, sodium polyphosphate, and the like. Such a preflushslug possesses better mobility control than an unthickened preflush slugand improves sweep efficiency by contacting, e.g., areas unswept bywater used in any previous waterflooding. Heterogeneous zones of variedand higher salinity would thusly be modified on the average to a lowersalinity range more suitable for practicing the instant process.

Aqueous Drive Fluid

An aqueous drive generally follows the permeability contrast correctionprocess of our invention. The aqueous drive employs available fieldbrines and/or fresh water if the latter is available.

The aqueous drive, since it follows our in-situ gelation treatment, isdiverted to the formerly relatively less permeable but still oil-richzones since the permeability contrast correction process slows orsubstantially prevents the flow of aqueous drive fluid through theformerly more permeable but now oil-poor zones (so-called thief zones).A successful permeability contrast correction operation generally issignaled at the production well by a reduction of the water/oil ratio inthe produced fluid.

Subsequent to the permeability contrast correction, the water/oil ratiomay gradually increase again after prolonged injection of the drivewater. A gelation retreatment of the formation can be consideredappropriate, if desired.

These gel-plugging techniques also are useful during well workovers, infracture treatments, and to correct the injection profile of a well byin depth sealing of communicating streaks of relatively highpermeability so that flooding fluids will enter the formation in a moreuseful front profile.

EXAMPLES

Examples are provided in order to assist one skilled in the art to afurther understanding of the invention. Particular species employed,exemplary facets, equipment, and the like, are designed to be furtherillustrative of the invention and not limitative of the reasonable scopethereof. The ratios of permeabilities (permeability contrast) in packsused in the examples was about 8-10 for Ottawa sand and about 5.8 forbeads. The use of the term "fresh water" in the Examples indicates theuse of Bartlesville tap water (BTW) or synthetic generator water (SGW).These "waters" were found to be essentially equivalent throughout theexperimental work. The synthetic generator water was prepared by addingthe following salts to 5 gallons (18.93 liters) of distilled waters:

    ______________________________________                                        NaHCO.sub.3       6.35 g                                                      NaCl              2.35 g                                                      Na.sub.2 CO.sub.3 0.65 g                                                      CaCl.sub.2 (anhydrous)                                                                          0.27 g                                                      MgCl.sub.2 (anhydrous)                                                                          0.23 g                                                      ______________________________________                                    

EXAMPLE I

This run demonstrates the feasibility of employing carbon dioxide toalter the pH of aqueous solutions. This run further demonstrates thatthe passage of gaseous CO₂ through an aqueous alkaline mixture iseffective to trigger the bulk gelation of an aluminumcitrate/polyacrylamide permeability contrast correction solution. Such asystem is known to gel over the pH range of 3 to 7. The CO₂ effectivelyreduced the pH of the mixture into the gelation range.

An aqueous aluminum citrate solution was prepared by mixing equalvolumes of aluminum sulfate and sodium citrate stock solutions. Thealuminum sulfate stock solution was prepared by dissolving 3.75 grams ofAl₂ (SO₄)₃.18H₂ O in sufficient distilled water to give 100 grams ofsolution (3.75 weight percent based on the aluminum sulfate 18-hydrate;approximately 0.11 molar in Al³⁺). The sodium citrate stock solution wasprepared by dissolving 1.1 grams of Na₃ C₆ H₅ P₇.2H₂ O in sufficientdistilled water to give 100 grams of solution (1.1 weight percent basedon the sodium citrate 2-hydrate; approximately 0.037 molar in citrate).The molar ratio of aluminum to citrate in the thusly prepared mixturewas about 3:1. The pH of the diluted mixture was adjusted to about 8.5by the addition of dilute (8 weight percent) aqueous sodium hydroxide.To the pH adjusted solution was added about 100 g of an aqueous 5,000ppm polyacrylamide solution (HF 1031 manufactured by Hercules Inc., andcharacterized as a very high molecular weight polyacrylamide, with20-30% hydrolyzable groups). The so-prepared permeability contrastcorrection slug contained 2,500 ppm polyacrylamide, and was about 0.055M Al³⁺ and about 0.0185 M citrate. After this addition of the aqueouspolyacrylamide, the pH of the solution remained at about 8.5 and therewas no evidence of gelation or precipitation.

On the passage of gaseous CO₂ through this mixture, the pH was loweredto about 6.3 and gelation became evident within 10 minutes.

EXAMPLE II

This run demonstrates that our inventive process effectively "correctedthe permeability" of a heterogeneous substrate simulated by a glassbead-packed tube containing zones of small diameter beads (lowpermeability zone) and coarse Ottawa sand (high permeability zone).

A sand zone was positioned centrally in a concentric tube system byplacing the sand in the smaller of the two concentric tubes and thenpacking the annular space between the two tubes with small glass beads(105-210μ diameter). The inner tube was then withdrawn. The glass beadsused were obtained from the Cataphate Division of the Ferro Corporation.The permeability of the centrally positioned sand zone was approximately9-10 times that of the peripheral zone of glass beads.

An aqueous polyacrylamide solution was prepared in Bartlesville tapwater to contain 5,000 ppm polyacrylamide (HF 1031). An aqueous aluminumcitrate solution was prepared by the procedure described in Example I togive a solution which was 0.11 molar in Al³⁺ and 0.037 molar in citrate.The pH of the aluminum citrate solution was adjusted to about 8.5 withaqueous NaOH (8 weight percent) before it was mixed with thepolyacrylamide solution.

A permeability contrast correction liquid slug was prepared by mixingequal volumes of aqueous polyacrylamide (5,000 ppm) with the pH-adjustedaqueous aluminum citrate solution, so that the slug contained 2,500 ppmof polyacrylamide, and was 0.055 M Al³⁺ and 0.0185 M citrate. Thegelable thickened solution was colored red with a water-soluble dye.Approximately 0.5 PV of the red-colored composition was passed into the"core". Then, CO₂ was passed into the packed tube until gasbreak-through occurred, and the apparatus was closed in with CO₂ at 70psi for 10 minutes. The pressure was released, and the CO₂ procedure wasrepeated including shut-in. Thereafter, one pore volume of fresh waterwas injected. The run was completed by injecting another pore volume offresh water colored with a water-soluble yellow dye.

In order to verify the effectiveness of our inventive process to"correct permeability" in the simulated heterogeneous matrix, the "core"material was sectioned at one-half inch intervals and thecross-sectional areas were examined. The central zone (highpermeability) was red in appearance showing the presence ofpolyacrylamide gelled with aluminum citrate. The peripheral zone (lowpermeability) was yellow in appearance showing that the final porevolume of fresh water (colored with the yellow dye) was diverted to theless permeable peripheral glass bead zone.

Thus, it was clearly demonstrated that the initially injected ungelledthickened aqueous slug passed predominantly into the centrallypositioned high permeability sand zone where gelation was triggered bythe subsequently injected CO₂ slug which fingered through the highpermeability streak to lower the pH therein into the gelation pH rangeof 3 to 7. The subsequently injected yellow-colored drive fluid wasdiverted into the lower permeability glass bead zone. FIG. 3 depicts thediversion of the aqueous drive slug through the lower permeability glassbeads.

EXAMPLE III

This run was carried out in essentially the same manner and in the sameapparatus as the run described in Example II, except that glass beadswere used to simulate the zones of both high and low permeability. Thecentrally positioned glass beads (simulating a high permeability streak)had diameters in the range of 105-210μ, and the peripherally locallyglass beads (simulating a low permeability zone) had diameters in therange of 53-105μ. The glass beads used were obtained from the CataphateDivision of the Ferro Corporation.

Observations and conclusions were similar to those disclosed in ExampleII. A longitudinal section of the "core" material was examined in thisrun. FIG. 3 depicts the diversion of the aqueous drive slug through thelower permeability glass beads.

EXAMPLE IV

This run demonstrates that merely the initial injection of an aqueouspolyacrylamide slug (without the inventive in-situ gelation step) intothe simulated heterogeneous matrix, using the sand/glass bead-packedconcentric tube apparatus as described in Example II, failed toeffectively divert the subsequently injected overflush water slug.

A 0.5 PV slug of a 2,500 ppm solution of polyacrylamide was injectedfollowed by an overflush treatment consisting of two consecutiveone-pore volumes of fresh water, the last slug of which was colored witha yellow dye.

After completion of the run, the core material was sliced longitudinallyand the centrally located Ottawa sand zone was seen to be yellow incolor. Thus, it was shown that the initially injected polymer solutionwas ineffective in decreasing the water permeability of the central sandpacked zone. Had the polymer slug effected a significant permeabilitycorrection, the yellow-colored overflush slug would have been divertedand the peripheral zone of the core material (lower permeability zone)would have appeared yellow in color. FIG. 2 depicts the preferentialpassage of all injected fluids through the centrally positioned highpermeability sand zone.

EXAMPLE V

The operability of the present invention was further demonstrated by theuse of parallel tubes packed, respectively, with large diameter glassbeads (simulating high permeability zone) and small diameter glass beads(simulating low-permeability zone). The coarse grade glass beads andfine grade glass beads had diameters, respectively, of 105-210μ and44-74μ.

The permeability contrast correction slug employed was prepared asdescribed in Example II. A volume of the slug was placed in a reservoirvessel and pumped into the parallel glass bead packed tubes. During theinjection of the aluminum citrate/polyacrylamide slug, 90 mL of liquidwas displaced through the coarse beads and 35 mL of liquid was displacedthrough the fine beads. Carbon dioxide then was injected until gasbreak-through was noted through the coarse bead pack. During the CO₂injection, 10 mL of fluid was displaced from the coarse bead pack andonly 2.5 mL of fluid was displaced from the fine bead pack.

This was followed by a fresh water overflush treatment with green-dyedwater. During the overflush treatment, only 85 mL of liquid wasdisplaced in the coarse glass bead pack, whereas 286 mL of liquid wasdisplaced from the tube containing the fine glass beads.

The relative amounts of liquid displaced from the coarse bead and finebead packs before and after the permeability correction treatmentclearly demonstrate and further verify the effectiveness of ourinventive process. The coarse (permeable) path was dominantly red incolor, while the fine pack (less permeable) was dominantly green afterthe overflush. From the foregoing data, it is evident that overflushliquid was diverted from the coarse glass bead pack to the relativelyless permeable fine glass bead pack.

EXAMPLE VI

The attached FIG. 5 reflects a summary of data illustrating the pHgelling range for a polyacrylamide/aluminum citrate system.

In order to show the effect of changes in pH on the crosslinkingbehavior of an aluminum citrate system containing an aluminum/citratemolar ratio of 1.6:1, solution samples were characterized by measurementof screen factors.

Screen factor is the ratio of time periods required for a specifiedvolume of a solution to pass through five 100-mesh stainless steelscreens in a viscometer as compared to the time required for the passageof an equal volume of pure solvent through the screens.

For the graph shown in FIG. 5, the solvent employed was brine (NaCl) andthe test solutions contained aluminum cations sequestered with citrateanions (molar ratio of aluminum to citrate 1.6:1) and polyacrylamide inaqueous salt brine media with pH adjusted over the range of 2 to 11.

A 1,200 ppm brine solvent passed through the viscometer in about 7seconds, and a 250 ppm solution of polyacrylamide (Dow Pusher 700) in1,200 ppm brine passed through in about 52.5 seconds, thus giving ascreen factor of about 7.5 (52.5 sec/7.0 sec).

Sample mixtures of aluminum citrate and of polyacrylamide wereformulated over a range of pH values (2 to 11) and then individuallypassed through the viscometer. Magnitude of the screen factors observedwere taken as a direct indicator of any crosslinking which had occurredin the sample, e.g., the greater the screen factor, the more extensivewas the crosslinking.

FIG. 5 shows that the screen factor peaks in the ranges of pH 3 to 7 andfrom 9 to 10 but for practical purposes, the effective gelation(crosslinking) pH range was 3 to 7 since the peak in the pH range of 9to 10 is so much smaller.

EXAMPLE VII

This Example demonstrates that the initial injection of an aqueous Na₂CO₃ slug into the bead pack can further delay the gelation of thesubsequently injected gelable composition. The experiment was carriedout in essentially the same fashion as described in Example II exceptfor the injection of 2 mL of a 1 weight percent aqueous sodium carbonateslug prior to the subsequent sequential injection of 70 mL of thealuminum citrate/polymer slug (colored with red dye), carbon dioxide and90 mL of fresh water (colored with yellow dye). Longitudinal sectioningof the bead pack after completion of the run showed that the red-coloredgelled polymer was localized in the centrally positioned highpermeability streak (large diameter beads) but farther from theinjection port than observed in Example II.

This Example illustrates that the inventive process can be controlled toeffect permeability contrast correction deeper in the formation by usingan initial injection of an alkaline slug such as aqueous Na₂ CO₃ toraise the pH near the well-bore to a pH level above the pH gelationrange. The gelation-triggering agent, i.e., the acidic CO₂, is therebyneutralized near the well-bore and becomes effective for triggeringgelation at a greater distance from the well-bore. FIG. 4 depicts thepositioning of the gelled composition in the high permeability zone butdeeper in the formation. FIG. 3 depicts the result of the inventiveprocess without the prior injection of Na₂ CO₃.

The disclosure, including data, has illustrated the value andeffectiveness of our invention. The examples, the knowledge andbackground of the field of the invention and the general principles ofchemistry and of other applicable sciences have formed the bases fromwhich the broad descriptions of our invention including the ranges ofconditions and the generic groups of operant components have beendeveloped, and formed the bases for our claims here appended.

We claim:
 1. A method for treating an underground formation whichcomprises:(A) injecting effective permeability control proportions of anin-situ gelable composition comprising effective ratios of:(a) water,(b) at least one polymer capable of gelling with a crosslinking agentwithin a gelation pH range, (c) at least one polyvalent metal cation,and (d) at least one sequestering anion; and thereafter (B) injectingcarbon dioxide, thereby causing gelation in-situ.
 2. The method of claim1 wherein said polymer is present in said gelable composition in anamount of about 500 to 10,000 ppm.
 3. A method according to claim 2wherein said polymer comprises polyacrylamide, the polyvalent metalcation comprises Al³⁺, and the chelating/sequestering anion comprisescitrate.
 4. The method of claim 2 employing in said in-situ gelablecomposition a range of about 10:1 to 1:10 molar ratio of polyvalentmetal cation to crosslinkable side groups of said polymer; and a molarratio of about 1:1 to 6:1 polyvalent metal cation:sequestering agent. 5.The method of claim 1 wherein said polymer is selected from the groupsconsisting of biopolysaccharides, cellulose ethers, acrylamide-basedpolymer, partially hydrolyzed polyacrylonitriles, polystyrenesulfonates, lignosulfonates, methylolated polyacrylamides, and mixtures.6. The method of claim 5 wherein said polyvalent metal cation isselected from the group consisting of Fe²⁺, Fe³⁺, Al³⁺, Ti⁴⁺, Zn²⁺,Sn⁴⁺, Ca²⁺, Mg²⁺, Cr³⁺, and mixtures; and said chelating/sequesteringagent is selected from the group consisting of acetate,nitrilotriacetate, tartrate, citrate, tripolyphosphate, metaphosphate,gluconate, phosphate, and mixtures thereof.
 7. The process of claim 6wherein said carbon dioxide is employed alone, in admixture with water,or diluted with nitrogen, steam, or mixture.
 8. The process of claim 7wherein said carbon dioxide is employed admixed with steam.
 9. Theprocess of claim 8 employing in said aqueous gelable composition about500 ppm to 10,000 ppm of said polymer.
 10. The process of claim 1preceded by an aqueous preflush slug to adjust connate water salinity.11. A process of controlling sweep efficiency in a subterraneanformation which comprises the sequential injection of effectivequantities of:(1) an aqueous preflush slug to adjust connate watersalinity, (2) an in-situ gelable aqueous thickened slug comprising asequestered polyvalent metal cation and a gelable polymeric viscosifier,(3) carbon dioxide in amounts sufficient to effectuate gelation of saidaqueous thickened slug; and (4) an aqueous drive fluid;thus effectingpreferential decrease in water permeability of the otherwise highlypermeable thief zones.
 12. The process of claim 11 wherein said aqueouspreflush comprises a sodium chloride brine.
 13. The process of claim 12wherein said in-situ gelable slug comprising effective ratios of:(a)water, (b) a polymer capable of gelling with a crosslinking agent at apH in the gelation range, (c) a crosslinking agent comprised ofsequestered polyvalent metal cations, selected from the group consistingof Fe²⁺, Fe³⁺, Al³⁺, Ti⁴⁺, Zn²⁺, Sn 4+, Ca²⁺, Mg²⁺, Mg²⁺, (d) chelatinganions selected from the group consisting of acetate, nitrilotriacetate,tartrate, citrate, and tripolyphosphate, metaphosphate, gluconate,phosphate, and mixtures thereof.
 14. The method of claim 13 wherein thepolymer is present in said gelable composition in an amount of about 500to 10,000 ppm.
 15. A method according to claim 14 wherein the polymercomprises partially hydrolyzed polyacrylamide, said multivalent metal isAl³⁺ and said chelating anion is citrate.
 16. The method of claim 14wherein said multivalent metal cation is present in said aqueous gelablesolution in a molar ratio of about 10:1 to 1:10 polyvalent metalcation:crosslinkable groups of said polymer, and said polyvalent metalcation is present in a molar ratio of about 1:1 to 6:1 polyvalent metalcation:sequestering anion.
 17. The process of claim 16 wherein saidpolymer is selected from the groups consisting of biopolysaccharides,cellulose ethers, acrylamide-based polymers, partially hydrolyzedpolyacrylonitriles, polystyrene sulfonates, lignosulfonates,methylolated polyacrylamides, and mixtures.
 18. The process of claim 17wherein said carbon dioxide employed as such, or in admixture with atleast one of air, nitrogen, steam, water, or mixtures thereof.
 19. Theprocess of claim 18 wherein said carbon dioxide is employed in admixturewith steam.
 20. The process of claim 19 employing in said aqueousgelable composition about 500 ppm to 10,000 ppm of said polymer.
 21. Theprocess of claim 20 wherein said polymer comprises polyacrylamide, saidpolyvalent metal cation comprises Al³⁺, and said chelating/sequesteringanion comprises citrate.